Prior transmitter and receiver circuits operating at high frequencies required that the data transmission system be interrupted or shut down in order to connect the transmitter and receiver circuits to the coaxial cable of the system, such as in U.S. Pat. No. 4,086,534 where the transmitting and receiving circuits are tapped into the transmission line by physically opening the line after the system has been shut down. In that patent, the transmitting and receiving circuits form a part of a distributed delay line which is used to connect them to the coaxial cable transmission line. The use of the distributed delay line for performing the connection has limitations with respect to data transmission rates and transmission line length. Ideally, the attachment of transmitter and receiver circuits should be made with zero loading of the transmission line at high frequencies and without interruption of system operation.
In the present invention, the transmitter and receiver circuits can be attached to the transmission line without interrupting the system. Also, the transmitter and receiver circuits can be attached to the transmission line with low loading of the line so as to enable the transmission of data at high frequency and the attachment of a large number of transmitter and receiver circuits. The transmitter and receiver circuits of the present invention are structured to present high impedance at the connection to the coaxial transmission cable. In the past, the transmitter and receiver circuits present too much capacitance at the connection to the transmission line and thus relatively low impedance. The lower impedance causes greater attenuation and thus a fewer number of transmitter and receiver circuits can be attached and the length of the transmission line is limited because of the losses caused by the attenuation. Also, the frequency or data rate of the transmission line becomes limited because of the losses caused by the attenuation.
The transmitter and receiver circuits of the present invention present high impedance at the connection to the transmission line whether powered on or off. This enables the use of stinger taps for connecting the transmitter and receiver circuits to the coaxial transmission line without interrupting the system operation. A stinger tap physically attaches to the coaxial cable by use of a clamp where two outboard terminals make contact with the shield of the coaxial cable and a central screwed needle, the stinger, pierces the insulation to contact the central conductor. The stinger tap is compatible with the present invention in that it also presents low capacitance, i.e. high impedance, at the connection so as not to load the transmission line.